A capacitor cell typically comprises an anode coated with dielectric, a separator, a cathode, and an electrolyte solution. The anode and cathode are both generally formed foils, and serve as the source of positive and negative charge, respectively. The energy of a capacitor cell is stored in the electromagnetic field generated by opposing electrical charges separated by the dielectric layer disposed on the surface of the anode. Etching may be used to increase the surface area of the anode, as the energy stored by the cell is proportional to the surface area of the anode. A dielectric oxide layer is formed on the anode when a voltage is applied in an electrolytic solution. The dielectric layer insulates the anode from the cathodic electrolytic solution, allowing charge to accumulate. The separator holds the anode and cathode foils apart to maintain charge and prevent short-circuiting. In one embodiment, the anode/separator/cathode laminate is typically rolled up to form a cylindrical body and encased, with the aid of suitable insulation, in an aluminum tube that is subsequently sealed with rubber material. In such embodiments it is imperative that the cathode foil and anode foil be precisely positioned opposite each other on the separator material. Such precise positioning of the anode and/or cathode on the separator material can be a difficult task to consistently achieve.
An alternative design, commonly used in implantable cardioverter-defibrillators (ICDs), typically comprise either flat electrolytic capacitor (FEC) cells or pressed, sintered and formed, powdered metal capacitors—presently available with a single tantalum anode member. These capacitors have been developed that overcome some disadvantages inherent in commercial cylindrical capacitors. For example, U.S. Pat. No. 5,131,388 to Pless et. al. discloses a relatively volumetrically efficient flat capacitor having a plurality of planar layers arranged in a stack. Each layer contains an anode layer, a cathode layer and means for separating the anode layers and cathode layers (such as paper). The anode layers and the cathode layers are generally comprised of foil plates of anode or cathode material and are usually electrically connected in parallel. In a paper “High Energy Density Capacitors for Implantable Defibrillators” presented at CARTS 96: 16 th Capacitor and Resistor Technology Symposium, Mar. 11–15, 1996, several improvements in the design of flat aluminum electrolytic capacitors are described, such as the use of an embedded anode layer tab and solid adhesive electrolyte. Further advances in flat electrolytic capacitors are found in U.S. Pat. 6,006,133, issued to Lessar et al., the disclosure of which is incorporated herein by reference.
For either flat or cylindrical capacitor cells, it is necessary that the anode and cathode remain separated. A minimum separation between the anode and cathode must be maintained to prevent arcing between the anode and cathode, and to allow charge to accumulate without short-circuiting. In cylindrical cells, the anode and cathode foils are aligned precisely with a separator positioned between them and coiled tightly to prevent movement of the anode, cathode and separator during subsequent processing and use. Spacing is typically maintained at the electrode edges as well by providing separator overhang at the top and bottom of the anode and cathode winding, to prevent short-circuiting to the casing. In flat capacitor cells, anode to cathode alignment is typically maintained through the use of internal alignment posts or screws (as described, for example, in U.S. Pat. No. 6,006,133 to Lessar et al.). Alignment of the anode and cathode plates in flat capacitor cells again can be somewhat problematic in that the plates are generally small and difficult to maneuver and maintain in position during assembly of the capacitor cell.
Maintaining a proper distance between capacitor components is thus one of the prime functions of a separator. A separator must be resistant to degradation, have sufficient thickness to maintain inter-electrode separation without interfering with cell high performance, and exhibit sufficient surface energy such that electrolyte wettability and absorption are augmented. The separator must also have an electrical resistivity sufficiently high to prohibit short circuit current from flowing directly between the electrodes through the separator. These requirements are balanced by the need for the separator to have porosity sufficient to maintain electrode separation while allowing ionic transfer to occur unimpeded within the electrolyte during discharge. Additionally, the separator must have sufficiently strong tensile properties to facilitate cell fabrication and to withstand internal cell stresses due to changes in electrode volume during discharge and re-charging cycles.
Separators are generally made from a roll or sheet of separator material, and a variety of separator materials have been found to be effective. Paper, particularly Kraft paper, is a cellulose-based separator material that is commonly used. Cellulose separator materials are manufactured with high chemical purity. The total thickness of cellulose separators employed between anode and cathode plates will vary with the voltage rating of the capacitor structure and the type of electrolyte employed but, on the average, the thickness varies from 0.003″ to 0.008″ in connection with capacitors rated at from 6 volts to 600 volts respectively.
A common alternative to paper separators are polymeric separators. Generally, polymeric separators are either made of microporous films or polymeric fabric. An example of a microporous film separator is a separator comprising polytetrafluoroethylene, disclosed in U.S. Pat. No. 3,661,645 to Strier et al. U.S. Pat. No. 5,415,959 to Pyszeczek et al., on the other hand, describes the use of woven synthetic halogenated polymers as capacitor separators. The use of “hybrid” separators comprising polymer and paper material has also been described. See, for example, U.S. Pat. No. 4,480,290 to Constanti et al., which describes the use of separators including a porous polymer film made from polypropylene or polyester and absorbent paper.
In the assembly of an FEC capacitor, it is important to maintain alignment of the anode, cathode, and separator components. Failure in either aspect can lead to short-circuiting or inefficient capacitor performance. In cylindrical capacitors, proper spacing is typically maintained at the electrode edges or peripheries by providing separator overhang at the top and bottom of the anode and cathode winding, which results in a larger capacitor than would otherwise be necessary. In addition, the anode and cathode are precisely aligned and coiled tightly by a winding machine to prevent movement of the anode, cathode, and separator during subsequent processing and use. In flat capacitors, anode to cathode alignment is typically maintained through the use of internal alignment posts. Build-up of static charge in the separator material during manufacture of the capacitor can made handling of the material particularly troublesome. All of these techniques have the disadvantage or requiring extra machinery or capacitor components that would not otherwise be required, among others. In the case of pressed, sintered and formed powdered metal capacitors, the alignment issues are not as great as with FEC cells, but the other desirable qualities for improved separator materials are still present.
It would thus be desirable to find a means to reliably and efficiently attach separator material to the electrodes of an electrolytic capacitor to reduce production costs and the likelihood of capacitor malfunction.